1. Field of the Invention
The present invention relates to a double-incline shaft kiln which calcinates limestone, dolomite, magnesite and other similar materials.
2. Description of the Prior Art
A type of conventional incline shaft kiln comprises a construction as is illustrated in FIGS. 1a and 1b. Ore carried by a belt conveyor is deposited into a charging hopper 1 at the top of the shaft kiln from which the ore is permitted to fall into an upper external portion of a shaft 3 via a charging mechanism 2. The material thus loaded into the shaft is preheated while descending through a preheating zone 11 by ascending hot gas. The preheated ore than passes through an upper calcining zone 12 where burning occurs by hot gas heated by an upper burner 4. The calcined materials further move down into a lower calcining zone 13 which is sloped in a direction opposite to that of the upper calcining zone 12 so that the calcined materials (ores) are turned over.
The calcined material is further roasted by a lower burner 5 in the lower calcining zone 13 and then falls down through the cooling zone 14 where it is cooled by cool air blowing in from an exhaust portion.
The calcined ore, thus cooled, is discharged out of the furnace by a piston-type discharging mechanism 6.
A portion of the hot gas is extracted from between the upper calcining zone 12 and the preheating zone 11 to preheat the combustion chamber in a recuperator 7. The remaining hot gas is used to preheat the material to be calcined. These hot gases are collected together by an exhaust gas fan 8 and passed through a dust collector 20 where they are cleansed of dust and then discharged into the open air.
Air to be used for combustion is fed by a blower 9 to the recuperator 7 where it is heated by the hot exhaust gas. The heated air is used for the burners 4 and 5 which are connected to a source of fuel (not shown).
The burners are located at the repose angle of the material and the upper and lower burning spaces 4a and 5a.
The upper burners 4, located in the upper burning space 4a, are operated at the ratio of theoretical combustion air of about 0.5 to roast the material using excess air from the lower burner.
The lower burners 5 are operated at the theoretical combustion air ratio of about 1.8, also using the cooling air.
The conventional double-incline shaft kiln of the above construction has the following drawbacks:
(1) As illustrated in FIG. 2, an inclined surface 12a is formed in the upper calcining zone 12 to turn over the ore. The speed at which the ore runs down the inclined surface 12a at the sidewall 12b is smaller than the speed of the material at the central portion of the inclined surface.
The time during which the ore stays on the inclined surface 12a differs beginning from the location thereof on the surface 12a, resulting in partial or uneven calcination of ore. In FIG. 2, the falling speed difference of ore is indicated by the superposed graphic illustration with the various velocities referenced v1.
(2) When solid fuels such as coals are used, they fall together with ore from the top of the furnace, mixing with the same, and burn using excess air contained in the ascending hot gas. The ore adjacent to the solid fuels are over-burned, resulting in partial or uneven calcination, and it is difficult to control the roasting condition.
As a result, ash of solid fuel fuses to the surface of calcined material resulting in coloring of the calcined material and formation of blocks. The ash of solid fuel mixes with the calcined material and thus deteriorates the quality of product.
(3) Because the calcining zone is short, a finish calcination cannot be done to those materials that are undercalcined, therefore, it is impossible to ensure products (calcined material) of good quality.
(4) Since the furnace is capable of calcining small grains of ore of about 5-30 mm in diameter, the gas pressure loss in the cooling zone 14 is large, which, in turn, reduces the amount of cooling air flowing in, thereby reducing the cooling effect on the product (calcined material) and reducing the heat recovery rate.